A flow channel device, a complex permittivity measuring apparatus, and a dielectric cytometry system are provided which can improve the measurement accuracy. A constriction portion having a constricted space is disposed between an inflow port and an outflow port of a flow channel. Electrodes are arranged between the inflow port and the constriction portion and between the outflow port and the constriction portion. The conductance of the constriction portion at a low-limit frequency is less than the combined conductance of an inflow channel portion and an outflow channel portion. The capacitance of the constriction portion at a high-limit frequency is less than the combined capacitance of the inflow channel portion and the outflow channel portion.